Heatpipes

[koshling]

I was hoping someone could explain something I don't understand about heatpipes to me.

My understanding was that heatpipes worked by transportation of heat using a low vapourisation temperature liquid (such as alcohol) to carry heat absorbed via the latent heat of vapourisation of the liquid in the pipe at the base of the heatsink, to the cooler part higher up where re-condensation occurs. This would mean that it is crucial for the base to be lower than the cooler regions (hence, presumably, the issues with inverted orientation of some ASUS MBs in some case).

However, the same issue would seem to apply to any mis-orientation close to 90 degrees or more. That seems to me to imply that most heatpipe based CPU coolers (I use a Zalman 9500 for example) should suffer serious degradation in cooling efficiency if the MB is mounted vertically (which make the heatpipes essentially horizintal). Since this is by far the most common orentation these days (i.e. - all towers/mini-towers), and these cooling solutions seem to work fine, I figure I must have misunderstood something...anyone care to enligten me please?

 

[Ayah]

Properly designed heatpipes will work in any direction. I'll let someone else do the long explanation thing since I don't know enough.

 

[tallman45]

Here you go

Heatpipe Defined

 

[Bill Brasky]

It depends on whether the heatpipe is wicked or not. Wicking draws in the condensed liquid and returns it to the other end of the heatpipe. Here is a quote from a website I found google searching.

"The heat pipes wick structure uses capillary pressure to pump the working fluid from the condenser to the evaporator. Typically, the restricting factor of heat pipe designs is the capillary limit, which is determined by the pumping capacity of the wick structure. The capillary limit is also a function of the heat pipes operating orientation and wick structure (e.g., groove, screen, cable/fiber, sintered powder metal). Pore radius and permeability are the two most important characteristics of a wick structure. Pore radius determines the pumping pressure that the wick can develop, while permeability determines the fractional losses of the working fluid as it flows through the wick."

Wicking Diagram

Text

Scroll down at little on the last link for an interesting table of what liquids are used for different temperature ranges.

 

[pcy]

Hi,


This is the heart of the problem.

Originally posted by: koshling
My understanding was that heatpipes worked by transportation of heat using a low vapourisation temperature liquid (such as alcohol) to carry heat absorbed via the latent heat of vapourisation of the liquid in the pipe at the base of the heatsink, to the cooler part higher up where re-condensation occurs.

Absolutley correct until the last phrase, where you say

[i.. at the base of the heatsink, to the cooler part higher up where re-condensation occurs.

the words base and higher up embody the idea that the circulation of material is driven by gravity.


Gravity may help, but it is not necessary.



Part of the problem that the heatpipe may be in a steady state, but it's a dynamic state none the less. Consider: if vapour did not move from the hot end to the cool end, the hot end would heat up even more, increasing the pressure and thus causing the fluid to move towards the cool end which has not (yet) heated up so much.


It not so obvious why the liquid shoud travel back.

The word "wick" is the answer here, though "surface tension" and "capillary action" are also involved. These words all represent the same idea - that a fluid tends to adhere to, and thus travel along, a surface, though to what extent depends on the properties of the fluid and of the surface.

This is a major aspect of the design of heatpipes.



Peter

 

[Bill Brasky]

That is a nicely worded reply, Peter. I kinda dove into the wicking head first without bothering to explain what it all means.

 

[Zap]

I think most CPU HSFs with heatpipes use wicking ones, but some motherboard chipset ones (like with Asus boards) don't, which means they don't work as well upside down (P180, Temjin, other cases).

 

[LOUISSSSS]

maximum pc doesn't recommend heatpipe cooled mb's be mounted upside down.
dec 2006, Vol 11. No 13 P 62

 

[pcy]

Hi Zaitsev,

Originally posted by: Zaitsev
That is a nicely worded reply, Peter. I kinda dove into the wicking head first without bothering to explain what it all means.

Thank you for saying so. Actually... your post helped me clear up my ideas about how the fluid does get returned, and that made my post much easier to write.



Peter